The present invention relates to brushless excitation systems for alternating current generators, and more particularly to protective means for protecting the rectifier diodes of such a system against reverse voltages.
Brushless excitation systems are frequently used for supplying direct current field excitation to large alternating current generators. Such an excitation system includes an alternating current exciter having a stationary field structure and a rotating armature member. A rotating rectifier assembly is carried on a common shaft with the exciter armature and is connected thereto to provide a direct current output. The output of the rectifier is supplied to the field winding of the main generator which is carried on the rotor and rotates with the exciter armature and rectifier assembly. In this way, an excitation system is provided which requires no commutator or slip rings and no sliding contacts.
Such excitation systems have many advantages and are widely used. Certain problems, however, may occur in the operation of these systems. For example, if the main generator loses synchronism and the rotor begins to slip poles, or if the rotor speed deviates from synchronous speed for any reason, the rotating magnetic field produced by the three-phase stator currents causes induced currents to appear in the field winding which are superimposed on the normal direct current excitation. If the amplitude of these induced currents is greater than that of the existing direct current excitation, the resultant current in the field winding will attempt to reverse and go negative as the induced current approaches its negative maximum. Since the field winding is connected across a rectifier bridge, the rectifier diodes block reverse current flow through the field winding. The reverse biased diodes, however, have a large impedance, and very high reverse voltages can occur across the rectifier bridge under these conditions. These voltages can be high enough to damage the diodes. The diodes should, therefore, be protected from dangerously high reverse voltages.